A spar is an integral component of many products. Such spars are generally load bearing structural elements used to add support to such products. For example, wing spars are provided in aircraft for fixing the wings to the aircraft and for transmitting lift generated by the wings to the fuselage of the aircraft.
It is known that spars are often formed using metal materials provided either as a solid or in a framework structure. For example, various metal spars are known for use in propeller blades [1-5] to provide structural support to airfoils that are formed thereabouts.
However, spars are sometimes now manufactured using composite materials (e.g. by using woven or braided materials of various fibres such as glass and/or carbon, etc.) because such composite materials [6-8] are generally lighter and stiffer than conventional metallic spars.
Nevertheless, whilst conventional composite material based spars are better for many applications they are not ideal, for example, where there is a need to form complex shapes with varying thicknesses, like for wing/propeller spars, etc.
Hence, to address the need to form complex spar shapes using composite materials, various manufacturing techniques have been adopted.
For example, when manufacturing a spar for a propeller it is known to use a ply-drop technique in which shaped portions of cut sheet cloth material are laid-up by hand in order to produce a stepped-edged approximation to an ideal three-dimensional shape of a spar. The stepped edge spar is then over-braided with composite fibre materials before being infused with resin to provide a finished airfoil-shaped propeller.
Although such a technique provides good quality propellers, there are certain disadvantages associated with such a manufacturing technique. For example, the cloth/fabric material is expensive and has to be quality inspected before it can be used, additionally the technique relies heavily on the skill and experience of an operator to correctly assemble the layers into the multi-layered stepped spar component. This also further slows the manufacturing process and makes production relatively expensive.
Additionally, the strength of the finished airfoil depends upon the adhesion of the individual shaped portions to one another in order to ensure that the individual layers forming the spar do not delaminate when in use. Furthermore, the stepped edge portions of the individual layers, or ply-drops, can introduce gaps between the multi-layered spar and the overlying braided skin where pockets of resin are prone to build-up in the finished propeller. These resin pockets can in turn create wrinkles in the spar of the propeller blade, in conjunction with braided fibre misalignment, causing subsequent possible creation of stress points where fractures in the structure may initiate.
It would therefore be desirable to provide an improved way of producing composite spars, particularly those having a varying thickness.